16.1. Primitive Data Types

In JavaScript, data types can fall into one of two categories: primitive or object types. A primitive data type is a basic building block. Using primitive data types, we can build more complex data structures or object data types.

While object types such as objects and arrays are mutable, primitive data types are immutable. Immutable data types are data types that cannot be changed once the value has been created.

Primitive data types include:

  1. Strings
  2. Numbers
  3. Booleans
  4. undefined
  5. null

16.1.1. undefined

undefined is a primitive data type in JavaScript which is assigned to declared variables, which have not been initialized.

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let x;
console.log(x)

Console Output

undefined

16.1.2. null

null is similar to undefined in that it represents an unknown value, however, it is assigned to values that the programmer wishes to keep empty.

let x = null;
console.log(x);

Console Output

null

16.1.3. Example

Let's say that we are still working for the zoo. We have objects created for animals like so:

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let giraffe = {
   species: "Reticulated Giraffe",
   name: "Cynthia",
   weight: 1500,
   age: 15,
   diet: "leaves"
};

Now, a new giraffe is coming to the zoo. We may want to initialize an object for the giraffe, but hold off on storing information in the weight property until the giraffe arrives. In this case, we could initialize the weight property like so:

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let giraffeTwo = {
   species: "Reticulated Giraffe",
   name: "Alicia",
   weight: null,
   age: 10,
   diet: "leaves"
};

This way, our object is properly initialized with all of the information we would need and we can update the weight property later when we have accurate information.

16.1.4. Check Your Understanding

Question

Which of the following are primitive data types? Mark ALL that apply.

  1. arrays
  2. Strings
  3. objects
  4. null

Question

Consider the following code block:

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let x;

console.log(x);

x is of what data type?

  1. null
  2. undefined
  3. NaN
  4. number